statistics-wise and counting only mages, than it's essentially a tie between Nalia and Imoen. their relevant stats (Int, Dex, Con) are the same while Nalia is stronger and Imoen has higher Wis and Cha. the sum of their stats is the same though.
however, Imoen does miss the early part of the game. that means she will always be lower level than Nalia if you had her from as early as possible. this means Nalia will always have the potential of being the more powerful mage of the two.
but then again, Imoen's thieving abilities are enough for her to be the only thief in any party, while Nalia's four thief levels are not nearly enough.